1. Field of Invention
The present invention relates generally to the field of marine seismic equipment, and specifically to position determination of marine seismic acoustic receivers, and estimations sound velocity profile in fluids.
2. Related Art
Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. In so-called seabed seismic, a cable containing seismic receivers is deployed onto the seabed from a surface vessel. The seismic operation needs accurate position determination of the receiver cable, and the typically used method for positioning is based on underwater acoustic ranging. Descriptions of so-called “base-line” positioning systems and methods may be found in textbooks and on the Internet. These include long-base-line (LBL), short-base-line (SBL), ultra-short-base-line (USBL), long-and-ultra-short-base-line (LUSBL); and even long-short-and-ultra-short-base-line (LSUSSL) systems and methods. The conventional way of determining the position of seabed seismic receiver cables is by use of USBL systems that are commercially available; however, USBL systems are not known to be used for estimating or calculating sound velocity profile of a fluid. Focusing on position determination, the main drawbacks with USBL systems are that they require transmitters to be mounted on the cable for every point to be tracked. This is a safety hazard for personnel working on the back deck on the deployment vessel, and it also reduces the maximum feasible speed of cable deployment and recovery. It is thus an economically impaired technique. Compared to the proposed techniques it is also less efficient in that the number of transmitters it is possible to fit on the cable is a number far less than the number of seismic receivers. The positions provided by the USBL system will only be at selected intervals and they do not coincide with the seismic receiver positions, so that one must interpolate based on the transmitter positions.
SBL systems generally utilize a transmitter mounted on the remote target, and the accuracy of the angular measurement provided comes from the use of sophisticated techniques for correlation of the observed signal at receivers located with a short baseline separation. The correlation aims at determining the difference in time of reception between pairs of receivers. The idea is that the signals observed at the two locations have the same source and are distorted in a similar way so that a good correlation can be found. The fact that the signal gets transmitted into the noisy area around the vessel hull is a disadvantage of this method as it poses further challenges on the sensing and processing of the signal.
From the above it is evident that there is a need in the art for improvement in positioning of marine seismic seabed cables and other seismic equipment.